In general, the cyclopentene carries out a metathesis ring-opening polymerization under the presence of so called Ziegler-Natta catalyst, which is made of a compound of transition metal belonging to Group 6 of a Periodic Table such as WCl6 or MoCl5 or so, and an organometal activating agent such as triisobutylaluminum, diethylaluminumchloride and tetrabutyltin or so; thereby an unsaturated linear polymer is provided, and it is known to exhibit excellent rubber characteristic and have characteristic as rubber for tires (Non-patent document 1). However, since it is a linear polymer, when used as the rubber for tires, depending on the rubber member constituting the tires, there was a problem that the handling stability declines such as a wet grip property or a dry grip property or so which are essential characteristics required. However, there is no known method to improve this.
On the other hand, the patent document 1 describes to carry out the ring-opening copolymerization of cyclopentene and a norbornene compound using a ruthenium carbene complex, thereby the copolymer having high elastic recovery rate can be obtained. However, if ruthenium carbene complex is used, the molecular weight does not increase, hence the weight average molecular weight (Mw) of the obtained copolymer is less than 100,000, and the molecular weight was too low to be used as the rubber for tires.
Also, recently, in connection with the demand to improve the fuel efficiency of the automobile, a low heat buildup property is demanded for the rubber for tires, and a dispersibility of carbon black or silica as the filler is demanded. As the method for this, the method which introduces the functional group such as amino group or alkoxysilyl group having high affinity with the carbon black or silica as the filler to the polymer terminal is considered. For the cyclopentene ring-opening copolymer, the method of introducing the functional group to the polymer terminal is considered, however the functional group cannot be introduced to the polymer terminal using the conventional Ziegler-Natta catalyst, hence the affinity against the filler was low and the dispersibility was poor. In connection with this problem, recently, the ruthenium carbene complex as the polymerization catalyst was developed, and when it was used as the polymerization catalyst, the functional group can be easily introduced to the polymer terminal of the cyclopentene ring-opening copolymer. Thus, in view of various purposes, the cyclopenete ring-opening copolymer wherein the functional group such as alkoxysilyl group, amino group, epoxy group, ester group, hydroxyl group, carboxyl group or so being introduced to the polymer terminal is synthesized (the patent documents 2 and 3). However, the weight average molecular weight (Mw) of the obtained cyclopentene ring-opening copolymer is less than 100,000, and the molecular weight was too low for the use as the rubber for tires.
Therefore, cyclopentene ring-opening copolymer which exhibits the wet grip property and the dry grip property is demanded when using the cyclopentene ring-opening copolymer as the component of the rubber composition for tires.